1. Field of the Invention
This invention relates to the field of integrated circuits comprising one or more conductances which are adjusted to a controlled value. Such circuits are useful in many applications, such as input-output couplers for high rate data buses, for example. Such buses are formed of high frequency lines which require matching impedances whose conductances are adjusted to very precise values. The integration of the conductances into the user integrated circuit offers a considerable space saving and reduces the problems of connecting the circuit to a calibrated component external to the integrated circuit.
2. Description of the Related Art
In order to avoid sensitivity of the adjustment of the conductance to electrical noise and to operational drift, it is preferable to drive this adjustment by means of a digital signal. A digital control signal is produced in such a way as to approximate the exact value desired using discrete values. The exact desired value is then included in an interval defined by two successive discrete values of the digital control signal. The size of this interval introduces an error in the control value relative to the exact desired value. The digital signal is constantly compared with the exact desired value, and therefore has a tendency to oscillate between the two discrete values which straddle the exact value. Now, oscillations of the adjusted value of the integrated conductance are unacceptable, since they keep modifying the current passing through them and accordingly interfere with its usage at high frequency.
One known solution consists in reducing the sensitivity of the comparison of the digital signal with the exact desired value by means of a dead band in which no account is taken of the comparison error to modify the digital signal. This dead band introduces a constant absolute error into the digital signal approximation of the exact desired value. Too small a width of the dead band relative to the size of the quantization intervals of the digital value makes the dead band inoperative for filtering out the oscillations. A width of the dead band greater than the quantization interval introduces an error greater than that of the interval alone. Now, in order to adjust the conductance with a constant relative accuracy, it is desirable to define the quantization interval as a constant proportion of the value of its lower limit. This results in small intervals for the low desired values and large intervals for the high desired values. A dead band width of the same order of magnitude as the smallest interval is inoperative for the high values to be adjusted and a dead band width of the same order of magnitude as the biggest interval introduces an error for the low values to be adjusted which is greater than the relative error obtained with the small intervals. A dead band which reduces the sensitivity of the comparison of the digital signal with the exact desired value therefore encounters difficulties for adjusting a digital signal with a constant relative accuracy.